The air traffic surveillance and control systems these days exploit the capabilities offered by the GNSS satellite positioning systems (GNSS being an acronym standing for “Global Navigation Satellite System”), such as, for example, the GALILEO, GLONASS and GPS systems. The aircraft to be surveyed and controlled usually comprise onboard means enabling them to finely evaluate their position by relying on these positioning systems. The aircraft then regularly send the result of their position estimation by radio to ground control stations. The ADS-B (Automatic Dependent Surveillance-Broadcast) system is one example of a standard for a surveillance system.
The earth stations are not always in radio visibility with an aircraft broadcasting its position, for example when the latter is moving over an ocean. The issue of deploying satellites that have capabilities for receiving and transmitting surveillance signals is then raised with the manufacturers in the sector because a satellite makes it possible to cover a wide area because of its altitude and its antenna aperture which can be greater than 126° at an altitude of 780 km. The use of a satellite would then have the advantage of improving the reception of the ADS-B type surveillance messages and therefore allowing for a finer control of the aircraft. In such a system, and as illustrated by FIG. 1, a plurality of aircraft 100, 101 transmit information such as, for example, their position, to a satellite 102 covering the area in which they are located. This transmission is performed by using a communication uplink 104. The satellite 102 then retransmits this information to at least one ground station 103 by using a downlink 105.
The use of satellites in an air traffic surveillance and control system does cause a number of problems. A first problem is due to the fact that the aircraft transmit surveillance signals asynchronously, and on one and the same frequency band. When the number of aircraft present in the area of visibility of the satellite becomes very high, the probability of messages colliding rapidly increases and may become prohibitive. A collision of signals occurs when two aircraft transmit at the same time and see their messages mutually interfered with. Furthermore, a solution that is based on a satellite covering a given area using a single spot is incompatible with an acceptable link budget, that is to say, with an adequate figure of merit. As a reminder, the figure corresponds to the ratio G/T between the antenna gain G and the temperature T. Furthermore, the onboard reception device in the satellite will easily find itself in a saturation situation because the number of airplanes in visibility is too great.
It is also possible to envisage a multi-spot solution, a satellite transmitting and receiving simultaneously in its coverage area using a number M of spots. Now, this solution presupposes the implementation, in the satellite, of as many reception subsystems as there are spots. It may then be difficult, or even impossible, to include such a receiver in the payload of a satellite because of its size, its volume, the number of equipment items, the power consumed and the weight of the reception device.